Sayyiduna Abdul Rahman bin Abza (may Allah be pleased with him) narrates from the Prophet (peace and blessings be upon him) that he (peace and blessings be upon him) used to recite this in the morning and in the evening:
'Asbahna 'ala fitratil-Islam - aw amsayna 'ala fitratil-Islam - wa 'ala kalimatil-ikhlas, wa 'ala dini nabiyyina Muhammadin (peace and blessings be upon him), wa 'ala millati abina Ibrahima hanifan musliman, wa ma kana minal-mushrikin'
"We have entered the morning upon the natural way of Islam, or we have entered the evening upon the natural way of Islam, and upon the word of sincerity, and upon the religion of our Prophet, Sayyiduna Muhammad (peace and blessings be upon him), and upon the religion of our forefather Sayyiduna Ibrahim (peace be upon him); we have entered the morning or evening upon it; he was an upright Muslim on the right path and was not of those who associate partners with Allah."
Ahmad and al-Tabarani recorded it, and the narrators of both are the narrators of the Sahih.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الأذكار / 17003
Hadith Gradingمحدثین:إسنادہ صحیح
Hadith Takhrij«اخرجه احمد فى المسند (406/3) ، أخرجه مسند أحمد برقم: 14935، 14938، وأخرجه سنن الدارمي برقم: 2688، وأخرجه مسند البزار برقم: 1911»