Al-Matalib al-'Aliyah 3726 — Surah Qaf

It is narrated from Sayyiduna Ali (may Allah be pleased with him) that he said: I asked the Prophet (peace and blessings be upon him) about this statement of Allah Almighty: "And at the setting of the stars" [Surah al-Tur: 49]. He (peace and blessings be upon him) said: "Adbar al-sujud, that is, 'And after the prostrations' [Surah Qaf: 40], refers to the two rak'ahs after the Maghrib prayer, and idbar al-nujum, 'at the setting of the stars', refers to the two rak'ahs before the morning (Fajr) prayer."
Imam al-Tirmidhi recorded this hadith in his Sunan (3275); its chain contains al-Harith bin Abdullah al-A'war, a weak narrator, but this same explanation is also transmitted in other reliable reports.