Allah, exalted be His praise, says: "And they were not commanded except to worship Allah, making religion sincerely His, inclining to the truth, and to establish prayer and give zakah; and that is the upright religion." [Surah al-Bayyinah: 5]
Along with it there are several other verses which mention the obligation of prayer.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحيض / Q1675
Ikrimah bin Khalid narrates a hadith to Tawus that a man came to Sayyiduna Ibn Umar (may Allah be pleased with them both) and said: O Abu Abdur Rahman! Why do you not go out on jihad? He said: I heard the Messenger of Allah (peace and blessings be upon him) say: "Islam is built upon five things: testifying that there is no god but Allah, establishing prayer, giving zakah, performing Hajj, and fasting in Ramadan."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحيض / 1675
It is narrated from Sayyiduna Uthman bin Affan (may Allah be pleased with him) that he (peace and blessings be upon him) said: "Whoever knows that prayer is a truth, obligatory, or prescribed, will enter Paradise."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحيض / 1676