Sahih al-Bukhari 743
Book 10: Call to Prayers (Adhaan) · Narrated by Anas bin Malik
Arabic
حَدَّثَنَا حَفْصُ بْنُ عُمَرَ، قَالَ حَدَّثَنَا شُعْبَةُ، عَنْ قَتَادَةَ، عَنْ أَنَسٍ، أَنَّ النَّبِيَّ صلى الله عليه وسلم وَأَبَا بَكْرٍ وَعُمَرَ ـ رضى الله عنهما ـ كَانُوا يَفْتَتِحُونَ الصَّلاَةَ بِ ـ {الْحَمْدُ لِلَّهِ رَبِّ الْعَالَمِينَ}
English translation
Narrated Anas bin Malik:The Prophet, Abu Bakr and `Umar used to start the prayer with "Al hamdu li l-lahi Rabbi l-`alamin (All praise is but to Allah, Lord of the Worlds)
Reference: Sahih al-Bukhari 743, Book of Call to Prayers (Adhaan). English translation by Dr. Muhammad Muhsin Khan.