Musnad Ahmad 1098:9

Yahya ibn Adam narrated to us, Ibn al-Mubarak narrated to us, from al-Awza’i, from Yahya ibn Abi Kathir, from Abu Salamah, and from ‘Ata’ ibn Yasar, from Abu Salamah, from ‘Abdullah ibn Salam, who said: We discussed among ourselves: "Which of you will go to the Messenger of Allah, may Allah bless him and grant him peace, and ask him: 'Which deeds are most beloved to Allah?'" None of us stood up. Then the Messenger of Allah, may Allah bless him and grant him peace, sent a man to us, so he gathered us, and he recited this Surah to us, meaning the entire Surah as-Saff.