Musnad Ahmad 1237:4

Yahya ibn Ishaq narrated to us, he said: Ibn Lahi'ah informed us, from Abdullah ibn Hubayrah, from Abu Tamim, from Abu Basrah al-Ghifari, who said: The Messenger of Allah (peace and blessings of Allah be upon him) led us in the Asr prayer in one of their valleys called al-Mukhammas. He said: "Indeed, this prayer—the Asr prayer—was presented to those before you, but they neglected it. Verily, whoever prays it, his reward shall be doubled for him twice. And verily, there is no prayer after it until you see the witness." I said to Ibn Lahi'ah: "What is the witness?" He said: "The star. The Bedouins call the star the witness of the night."