When should the hadd punishment for zina be carried out on a man?

The hadd punishment for zina is incurred when the head of the penis enters the vagina. In that case the two circumcised parts have met, i.e., the site of the man’s circumcision and the site of circumcision in the female. If penetration takes place, then this is the zina which incurs the hadd punishment, whether the man ejaculates or not, or if he penetrates to this extent then ejaculates outside, and whether his penis is erect or not. 

It says in al-Mawsoo’ah al-Fiqhiyyah (24/23), explaining the conditions of the hadd punishment for zina on which the fuqaha’ are agreed: There is no difference of opinion among the fuqaha’ that in order for the hadd punishment to be incurred, the head of the penis has to disappear into the vagina. If it does not enter it at all or only part of it enters, then there is no hadd punishment because that is not intercourse. There is no stipulation that ejaculation must take place or that the penis must be erect at the time of penetration. The hadd punishment must be carried out whether he ejaculated or not, and whether his penis was erect or not. End quote. 

Secondly: 

This does not mean that a person may be negligent about committing haraam actions so long as they do not reach the level of zina! Rather the intention here is simply to define the zina that incurs the hadd punishment. Being alone with a non-mahram woman, touching her or kissing her are all undoubtedly haraam, and the Muslim must avoid such actions for fear of the punishment of Allaah sooner or later, in this world or in the Hereafter. 

We ask Allaah to guide us and make us pious and chaste. 

And Allaah knows best.