It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) used to encourage the night prayer (Tahajjud) until he (peace be upon him) said: "Even if you pray only one rak'ah during the night."
Hadith Referenceسنن دارمي / من كتاب الرقاق / 2757
Hadith Gradingتحقیق (حسین سلیم أسد الدارانی):إسناده ضعيف جدا من أجل حسين بن عبد الله بن عبيد الله. ولكن الحديث صحيح لغيره، [مكتبه الشامله نمبر: 2764]
Hadith Takhrijاس روایت کی یہ سند ضعیف ہے، لیکن حدیث صحیح لغیرہ ہے۔ دیکھئے: [مجمع الزوائد 3565، بتحقيق حسين دراني]
Brief Explanation
(Commentary on Hadiths 2755 to 2757)
From this hadith, the importance of tahajjud or qiyam al-layl (standing in prayer at night) is understood.
Also, it is established that performing witr as a single rak‘ah (unit of prayer) is permissible, as is mentioned in another authentic hadith: If you fear that dawn is approaching, then perform one rak‘ah. Therefore, to say that a single rak‘ah does not constitute a prayer is incorrect.
Once, this humble one saw in a dream that Samahat al-Shaykh Ibn Baz rahimahullah was emphasizing the importance of tahajjud. When I mentioned this dream to him, he said: Qiyam al-layl is not obligatory (wajib), but one should certainly perform prayer at night, even if it is only two rak‘ahs.