Musannaf Ibn Abi Shayba 3:2958

Hushaym narrated to us, he said: Mughira informed us, from Ibrahim, who said: Abu Ma‘mar said, "There is a matter I wished I had asked al-Aswad about." Ibrahim said, "And what is it? Perhaps you have been spared [the need to ask] it." He said, "A man who catches one rak‘ah of the Jumu‘ah [prayer]." He said: Al-Aswad said, "Whoever catches one rak‘ah of the Jumu‘ah, let him pray another rak‘ah. And whoever does not catch the bowing (ruku‘), let him pray four."