Legal Rulings and Conditions for Supplications During the Prayer
Any supplication would be fine there. Your past prayers are valid, Alhamdulillah.
The condition for valid supplication in prayer is that it either be (a) with something transmitted in the Qur’an and Sunna, or (b) something not resembling human speech–namely, something that one cannot ask other humans for. [ Ibn Abidin, Radd al-Muhtar; Shurunbulali, Maraqi al-Falah ] Thus, asking Allah that, “O Allah marry me to Zaid/Zubaida” within the ritual prayer would invalidate the prayer, as this is something one could ask another human for.
However, it isn’t a condition that the supplication be specifically established in the Qur’an and Sunna. The reason for this is that the Divine Command, “Call upon Me, and I shall answer you,” is general, and can be fulfilled. This was conditioned, when it comes to ritual prayer, by the words of the Messenger of Allah (peace and blessings be upon him), “No human speech befits this prayer of ours.” [Related by Muslim, on the authority of Mu`awiya ibn al-Hakam (Allah be pleased with him), and others]
The legal rulings of supplication (du`a’) in prayer:
(1) A confirmed sunna, in the final sitting before the final salams.
(2) Recommended, during prostration in supererogatory prayers.
(3) Good, during prostration in obligatory prayers for other than the imam.
(3) Good, to avoid the difference of opinion of those who deemed it necessary, between the two prostrations, by seeking forgiveness.
[ Ibn Abidin, Radd al-Muhtar ]
And Allah alone gives success.
Answered By: Shaykh Faraz Rabbani