Al-Matalib al-'Aliyah 209 — The virtue of prayer

It is narrated from Abu Qilabah that Sayyiduna Umar (may Allah be pleased with him) came to Sayyiduna Mu'adh bin Jabal (may Allah be pleased with him) while he was among a group of his companions, and he (may Allah be pleased with him) asked: "O Mu'adh, on what do the foundation, pillar and stability of this matter (that is, the religion of Islam) rest?" He replied: "On sincerity (ikhlas) for Allah's sake, and this is the sound natural disposition (fitrah) (upon which Allah created mankind); and on the prayer, and this is the fundamental mark of the Muslim community (millah); and on obedience and submission to Allah and His Messenger."
Imam Musaddad bin Musarhad narrated this report (athar) in his Musnad. All its narrators are thiqat (trustworthy); however, since Abu Qilabah's meeting with Sayyiduna Umar (may Allah be pleased with him) is not established, it is munqati' (broken) in its chain (mursal), but it has supporting reports in other books of hadith.