The Prophet ﷺ said, "Prayer is performed two units at a time. You make the testimony after every two units, show humility and submission, raise your hands in supplication, and say, 'O Allah, O Allah.' Whoever does not do this, his prayer is deficient."
حدثنا ابن المثنى حدثنا معاذ بن معاذ حدثنا شعبة حدثني عبد ربه بن سعيد عن أنس بن أبي أنس عن عبد الله بن نافع عن عبد الله بن الحارث
عن النبي ﷺ قال الصلاة مثنى مثنى أن تشهد في كل ركعتين وأن تباءس وتمسكن وتقنع بيديك وتقول اللهم اللهم فمن لم يفعل ذلك فهي خداج
[Its chain of transmission is daif] Ibn Majah narrated it in the Book of Establishing the Prayers, Chapter on What Has Been Reported Concerning the Night and Day Prayer Being Two by Two, hadith 1325, from the hadith of Shuba, with this wording. Abu Hatim Razi graded it hasan in his Ilal, 365. Ibn Khuzayma, hadith 1212, indicated its weakness, and Bukhari and others graded it daif, which is the more correct view. There is doubt regarding whether Abdullah bin Nafi heard directly from Abdullah bin Harith, and the chain contains other defects as well.