The Prophet (peace and blessings be upon him) said:
"Indeed, whoever stands (in the night prayer) with the imam until the imam finishes (completing the prayer), the reward of standing in prayer for the whole night is recorded for him."
Hadith Referenceصحیح الجامع الصغیر / حرف الألف / 2417
Hadith Gradingالألبانی:صحيح
Hadith Takhrij[ت هـ حب] عن أبي ذر. الإرواء ٤٤٧، صلاة التراويح ص ١٤.