The conditions for a hadith to be classified as Sahih (sound/valid) are as follows:
1. Continuity of Chain or Sanad: Every transmitter should be joined to the next successive transmitter directly without any missing link between them throughout the whole chain of transmission, in other words it should be Marfu Mutasil, what is also called Musnad.
2. Piousness and Taqwa of transmitter: Every transmitter (Raawy) should be extremely pious, mature with sound mind and an honoured person.
3. Memory power or written documentation: Every narrator should have extraordinary memory power so that it can be assumed that whatever Hadith he has learnt, he remembers it well or he should have got properly written Hadith from reliable sources.
4. Lack of opposition to known authorities of Hadith: No Raawy or transmitter should have opposed the known authorities of Hadith or a Muhaddith superior to him.
5. Lack of reasons which could give rise to any doubt; Raawy should not have any such ailment, apparent or hidden, which can tell upon his health.