Sayyidah Aishah (may Allah be pleased with her) said: How strange is the Muslim man who, when he enters the Ka'bah, raises his gaze towards the ceiling! He should leave that out of reverence and veneration for Allah. The Prophet (peace and blessings be upon him) entered the Ka'bah, and he (peace and blessings be upon him) did not move his gaze from the place of his prostration until he came out of it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9726