It was said in Falaahi Ssa'il that the most important supplication after (Zuhr) noon prayer is to imitate Imam Sadiq (peace be upon him) in his supplication for Imam Al-Mahdi (peace be upon him) which the Prophet has given his glad tidings to his nation in the authentic narrations, Prophet has promised his nation that Imam Al-Mahdi will appear prior to the last hour as Muhammad bn Rahban Dabili has reported:
On the authority of Ibad bn Muhammad Mada'in, he said: I entered to Abu Abdullah in Medina when he finished his noon prayer and I met him raising his hands to the Heaven offering the following supplication:

بِسْمِ اﷲِ الرَّحْمٰنِ الرَّحِیْمِ

Bismillaahir-Rahmaanir-Raheem

In the name of Allah the Beneficent the Merciful.

يَا سَامِعَ كُلِّ صَوْتٍ، يَا جَامِعَ (كُلِّ فَوْتٍ)، يَا بَارِئَ كُلِّ نَفْسٍ بَعْدَ الْمَوْتِ، يَا بَاعِثُ، يَا وَارِثُ، يَا سَيِّدَ السَّادَةِ، يَا اِلٰهَ الْاٰلِهَةِ، يَا جَبَّارَ الْجَبَابِرَةِ، يَا مَلِكَ الدُّنْيَا وَالْاٰخِرَةِ، يَا رَبَّ الْاَرْبَابِ۔ يَا مَلِكَ الْمُلُوْكِ، يَا بَطَّاشُ، يَا ذَا الْبَطْشِ الشَّدِيْدِ، يَا فَعَّالًا لِمَا يُرِيْدُ، يَا مُحْصِيَ عَدَدِ الْاَنْفَاسِ وَنَقْلِ الْاَقْدَامِ، يَا مَنِ السِّرُّ عِنْدَهُ عَلَانِيَةٌ، يَا مُبْدِئُ، يَا مُعِيْدُ۔

Ya sami'a kulli sawtin, ya jami'a (kulli fawtin), ya bari'a kulli nafsin ba'da al-mawt, ya ba'ithu, ya warithu, ya sayyid as-sadati, ya ilaha al-alihat, ya jabbar al-jabbarah, ya malika ad-dunya wal-akhirah, ya rabb al-arbab. Ya malik al-muluk, ya battash, ya dha al-batsh ash-shadid, ya fa''alan lima yurid, ya muhsiya 'adad al-anfas wa naqli al-aqdam, ya man as-sirru 'indahu 'alaniyah, ya mubdi', ya mu'id.

O He who hears all voices, O he who gathers everything, O he created soul after death, O the causer, O the inheritor, O the master of masters, O God of gods, O The Mighty over the oppressors, O the king of this world and the hereafter, O the Lords of the lords, O the King of the kings, O the O He who does what he wishes, O he who knows the number of breaths and the movement of feet, O He to whom all secrets are visible, O the starter, O He who returns everything.

اَسْاَلُكَ بِحَقِّكَ عَلٰى خِيَرَتِكَ مِنْ خَلْقِكَ، وَبِحَقِّهِمُ الَّذِيْ اَوْجَبْتَ لَهُمْ عَلٰى نَفْسِكَ، اَنْ تُصَلِّيَ عَلٰى مُحَمَّدٍ وَاَهْلِ بَيْتِهِ (عَلَيْهِ وَعَلَيْهِمُ السَّلَامُ)، وَاَنْ تَمُنَّ عَلَيَّ السَّاعَةَ السَّاعَةَ بِفِكَاكِ رَقَبَتِيْ مِنَ النَّارِ، وَاَنْجِزْ لِوَلِيِّكَ وَابْنِ نَبِيِّكَ الدَّاعِيْ اِلَيْكَ بِاِذْنِكَ، وَاَمِيْنِكَ فِيْ خَلْقِكَ، وَعَيْنِكَ فِيْ عِبَادِكَ، وَحُجَّتِكَ عَلٰى خَلْقِكَ، عَلَيْهِ صَلَوَاتُكَ وَبَرَكَاتُكَ وَعْدَهُ۔

As'aluka bihaqqika 'ala khayratika min khalqika, wa bihaqqihimu alladhi awjabta lahum 'ala nafsika, an tusalli 'ala Muhammad wa ahli baytihi (alayhi wa 'alayhim as-salam), wa an tamunna 'alayya as-sa'ata as-sa'ata bifikaki raqabati min an-nar, wa anjiz li waliyyika wa ibn nabiyyika ad-da'i ilayka bi-idhnik, wa ameenika fi khalqika, wa 'aynaka fi 'ibadika, wa hujjatuka 'ala khalqika, 'alayhi salawatuka wa barakatuka wa 'adduhu.

I requested from You by the sake of your right over the best among your creatures and for the sake their right you have made incumbent on Yourself to send Your blessing on Muhammad and members of his house and to grant me the rescue of my neck from Hellfire, execute for your good friends from the descendant of Your Prophet, the callers to Your course with Your permission, Your trustee among Your creatures, Your sight among Your servants, Your proof among Your creatures, may Your peace and blessing be upon him.

اَللّهُمَّ أَيَّدَهُ بِنَصْرِكَ، وَانصُرُ عَبْدَكَ، وَ قَوِّ أَصْحَابَهُ وَ صَبِّرْهُمْ، وَافْتَحْ لَهُمْ مِنْ لَدُنْكَ سُلْطَانًا نَصِيرًا، وَ عَجِّلْ فَرَجَهُ، وَ أَمْكِنْهُ مِنْ أَعْدَائِكَ وَأَعْدَاءِ رَسُولِكَ يَا أَرْحَمَ الرَّاحِمِينَ;

Allahumma ayyidhu bi-nasrika, wa ansur 'abduka, wa qawwi ashadahu wa sabbirhum, waftah lahum min ladunka sultanan nasiran, wa 'ajjil farajahu, wa amkinhu min a'da'ika wa a'da'i Rasulika ya arham ar-rahimin.

O Allah! Support him with Your assistance and assist your servant, empower his companions and grant them patience, open for them a protected authority and hasten his relief, let him prevail over Your and Your Prophet's enemies, O the most merciful of the merciful.

I said to Imam: May my soul be sacrificed for you, isn't you have prayed for yourself? He said: I have prayed to the light of the progeny of Muhammad (peace be on them) who shall retaliate against their enemies with the command of Almighty Allah.
I said: when will he be coming out?
When the owner of creations and command wishes, Imam replied. I said: what are the signs before then? He said: the signs are many and different. I said: like what? Imam Sadiq (peace be upon him) said: A banner shall come out from the east and west, sedition will persist and a man shall come out from Yemen among the descendant of my uncle Zaid, and there shall be plunding of the house properties then Allah will act according to his wishes.