Bg 2.15

yam hi na vyathayanty ete
purusam purusarsabha
sama-duhkha-sukham dhiram
so ’mrtatvaya kalpate

Word for word: 
yam — one to whom; hi — certainly; na — never; vyathayanti — are distressing; ete — all these; purusam — to a person; purusa-rsabha — O best among men; sama — unaltered; duhkha — in distress; sukham — and happiness; dhiram — patient; sah — he; amrtatvaya — for liberation; kalpate — is considered eligible.

Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
O best among men [Arjuna], the person who is not disturbed by happiness and distress and is steady in both is certainly eligible for liberation.

Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Anyone who is steady in his determination for the advanced stage of spiritual realization and can equally tolerate the onslaughts of distress and happiness is certainly a person eligible for liberation. In the varnasrama institution, the fourth stage of life, namely the renounced order (sannyasa), is a painstaking situation. But one who is serious about making his life perfect surely adopts the sannyasa order of life in spite of all difficulties. The difficulties usually arise from having to sever family relationships, to give up the connection of wife and children. But if anyone is able to tolerate such difficulties, surely his path to spiritual realization is complete. Similarly, in Arjuna’s discharge of duties as a ksatriya, he is advised to persevere, even if it is difficult to fight with his family members or similarly beloved persons. Lord Caitanya took sannyasa at the age of twenty-four, and His dependents, young wife as well as old mother, had no one else to look after them. Yet for a higher cause He took sannyasa and was steady in the discharge of higher duties. That is the way of achieving liberation from material bondage.