Sayyiduna Abu Musa al-Ash'ari (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "Two or more people are a congregation."
Ulaylah, who is al-Rabi' bin Badr, is da'if (weak); and it has also been narrated through another weak route.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5008