Musa bin Salamah says: I asked Sayyiduna Ibn Abbas (may Allah be pleased with them both) about the prayer at al-Abtah, and he said: "Two rak'ahs; this is the Sunnah of Abu al-Qasim (peace and blessings be upon him)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12840