Musannaf Ibn Abi Shaybah 6823 — Those who said: if you repeat Maghrib

Masruq was asked: If a man prays Maghrib alone and then repeats it in congregation, how should he perform it? He said: He should add one rak'ah to it.
Sayyiduna Ali (may Allah be pleased with him) said: He should make it even with one rak'ah, meaning when he repeats Maghrib.