Sayyiduna Abu Bakrah (may Allah be pleased with him) reports that the Prophet (peace and blessings be upon him) said: "Ask Allah Almighty with your palms! Do not ask Him with their backs (that is, while supplicating keep the palms facing upwards, do not keep the backs of the palms upwards)."
Al-Tabarani recorded it, and its narrators are the narrators of the 'Sahih', except Ammar bin Khalid al-Wasiti, and he is thiqah (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الأدعية / 17346