Abdullah bin Dinar (may Allah have mercy on him) said that Abdullah bin Umar (may Allah be pleased with them both) wrote to Abd al-Malik bin Marwan, who was asking him for his pledge of allegiance: "Bismillah al-Rahman al-Rahim (In the name of Allah, the Most Gracious, the Most Merciful). Amma ba'd (to proceed): To Abd al-Malik, the Commander of the Believers, from Abdullah bin Umar (may Allah be pleased with them both). Salamun alayka (Peace be upon you). I praise Allah, besides whom there is no god, and I affirm (my commitment) to hear and obey, according to the Sunnah of Allah and the Sunnah of His Messenger (peace and blessings be upon him), as far as I am able."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قتال أهل البغي / 16564