We know that we are happy when our mind is peaceful, and unhappy when it is not. It is therefore clear that our happiness depends upon our having a peaceful mind and not on good external conditions. Even if our external conditions are poor, if we maintain a peaceful mind all the time we shall always be happy.
Modern Buddhism: The Path of Compassion and Wisdom - Volume 1 Sutra (ed. Tharpa Publications US, 2011) - ISBN: 9781906665852