Allah, exalted be His praise, said in praise of a people: "They fulfil their vows and fear a Day whose evil will be widespread." [Surah al-Insan: 7]
And He said in condemnation of others: "And among them are those who made a covenant with Allah: 'If He gives us of His bounty, we will surely give charity and surely be among the righteous.' But when He gave them of His bounty, they were stingy with it and turned away, and they are averse. So He put hypocrisy into their hearts as a consequence, until the Day they will meet Him, because they broke the promise they had made to Allah and because they used to lie." [Surah al-Tawbah: 75-77]
Sayyiduna Abdullah bin Amr (may Allah be pleased with them both) narrates from the Prophet (peace and blessings be upon him) that he (peace and blessings be upon him) said: "Whoever has four things in him is a pure hypocrite. Whoever has one of the four has one trait of hypocrisy until he abandons it: (1) when he speaks, he lies; (2) he breaks his promise; (3) when he disputes, he resorts to abuse; (4) he breaks his covenant."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / 20087