Capital gain tax

Dear Sir, Kindly suggest in the following case. In a partnership firm two sons and father are the partners. The property of the firm is in the name of firm. A partner (son) is retired from the firm and according to family settlement his share is to go to the second  son i.e. partner. Is there any capital gain arises? anytype of tax liability? what documents need to transfer the share of one partner to other in question? What will be the procedure ?