My brother led Thuhr prayer and he started to recite Surah Faatihah out loud in the first Raka'h but after saying Iyyaka Na'budu he stopped, and then I started to correct him, but then I too realized that I was making a mistake. At the end of the Salat he did not make Sajda-e-Sahw. Is the Salat valid or do we have to make Qadha?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

Whoever prays a silent prayer [e.g. Thuhr or 'Asr] loudly or a loud prayer like Maghrib or 'Ishaa' silently, his prayer is valid. The majority of the jurists are of the view that performing a loud prayer loudly, or a silent prayer silently, is a recommended act of the prayer, and whoever does not observe this out of forgetfulness is not obliged to prostrate for forgetfulness according to the majority of scholars, like Shaafi'ee and Awzaa'ee and one of the narrations of Imaam Ahmad, may Allaah have mercy on all of them. Therefore, your prayer is valid and you do not have to prostrate for forgetfulness.

Allaah Knows best.