Sometimes I am lazy in performing prayers. Is it permissible for me to do prayers by sitting at that time?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

There are two cases for your question.

The first case: If this is in regard to supererogatory prayers, then it is permissible to pray them while sitting; however, the reward is half of that praying standing.

The second case: If this is in obligatory prayers; this is not permissible unless you are unable to perform the prayer while standing. Standing in obligatory prayers is a pillar without which the prayer is invalid unless a person is unable to pray standing. 'Imraan Ibn Husayn narrated that he had haemorrhoids and he asked the Prophet how to perform the prayer; the Prophet told him: "Pray standing, if you cannot do so, pray sitting, and if still you cannot do so, pray lying on one of your sides." [Al-Bukhari]

Based on the above, you should know that praying an obligatory prayer in a sitting position because of laziness is forbidden and is therefore invalid.   

Allaah Knows best.