Ibn Ishaq relates the sermon of Sayyiduna Abu Bakr (may Allah be pleased with him) on that day, in which he (may Allah be pleased with him) said: It is not permissible for the Muslims to have two leaders, for then their commands and judgements would differ, division would spread among the Muslims and they would dispute among themselves; the Sunnah would be abandoned, innovation would appear, and trials would rise up, and there is no good in that for anyone.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قتال أهل البغي / 16550