A downhole tool, which includes a tool housing having a longitudinal axis, is equipped with a sting for punching a hole in a casing wall and injecting a sealant through the hole. The tube has a fluid channel to establish fluid communication from within the tool housing to an exterior of the tool housing through the fluid channel. A press device acts on the sting to force the sting in a radially outward direction from the tool housing. A check valve is arranged in the fluid channel, which allows fluid communication in a direction from within the tool housing to an exterior of the tool housing and which blocks fluid flow in an opposite direction. In use, the sting can perforate a casing wall and the sealant can be injected into an annular space around the casing.