Anas bin Malik reported,

The Prophet ﷺ, Abu Bakr, and Umar رضي الله عنهم used to begin the prayer with "All praise belongs to Allah, Lord of the worlds" [Fatihah 1:2].

Azami 1959

Sahih

عن أنس قال

إن النبي يا الله ﷺ وأبا بكر وعمر رضي الله عنهما كانوا يفتتحون الصلاة ب {الحمد لله رب العالمين}.

الأعظمي 1959صحيح
Contents
Reference
Commentary
Ayat
Collection
Jami al-Kamil
Book
Prayer
Chapter
Description of prayer, the takbir, standing, and recitation
Section
Beginning with Fatihah before the surah
Numbering
Jami al-Kamil 1959 (2nd edition)
Azami 1959