Sunan An Nasai
1053
It was narrated from Muhammad bin Maslamah that:
When the Messenger of Allah (ﷺ) stood to offer a voluntary prayer, he would say when he bowed: Allahumma laka rak'atu wa bika amantu wa laka aslamtu wa alayka tawwakaltu, anta Rabbi, khasha'a sam'i..
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