Musnad Abu Ya'la al-Mawsili 252:41

Harun ibn Ma'ruf narrated to us, Ibn Wahb narrated to us, he said: And 'Ayyash ibn 'Uqbah al-Hadrami narrated to me, that Yahya ibn Maymun al-Hadrami narrated to him, he said: Sahl ibn Sa'd al-Ansari passed by me while I was sitting in the mosque next to the Maqsura. He said to me: "Shall I not inform you of what I heard from the Messenger of Allah, may Allah bless him and grant him peace?" I said to a man beside me—there was nothing between him and the Messenger of Allah, may Allah bless him and grant him peace, except for this—"Yes, may Allah set you right, so inform me." He said: "I heard the Messenger of Allah, may Allah bless him and grant him peace, say: 'Whoever is in the mosque waiting for prayer, he is in a state of prayer as long as the prayer detains him.'"