Patañjali's yogasūtra 1:23 Īśvara-praṇidhāna

In the last four sūtras we learned that samādhi is a result either of birth, certain qualities, the intensity of the practioner's desire as well as of the practices employed. Sūtra 1:23 is what is known in common parlance as a 'game-changer'. Yoga is based on sāṃkhya, which posits two ontological principles: the sentient puruṣa (spiritual) and the insentient prakṛti (material). There is no room for 'god' in this philosophical system* However yoga, allows for an Īśvara, complete surrender to whom can bring about or hasten the achievement of samādhi.

The Bihar School, Munger is at pains to explain that Īśvara is a generic term and is not limited to a Hindu god. You could focus your attention on an image of the baby Jesus, or a picture of Mecca Sharif as effectively as focusing on a shiva-linga, or an idol of Krishna, Rama, Ganesha, Durga etc. Going forward we will see that Patanjali allows various ways to center yourself and bring about single-pointedness (ekāgratā) in the mind. You can choose from a variety of methods depending on what suits your personality. For instance concentrating on a tantric yantra works for one, verbal recitation or silent contemplation of a mantra for another. Yoga is all about यथा शक्ति and यथा भाव. 

{1:23}

ईश्वर प्रणिधानाद्वा 

Or, [samādhi can be achieved] as a result of fixing one's mind on the Lord

ईश्वरः प्रणिधानात् वा 

ईश्वरः - Lord, Master, Supreme Being

प्रणिधानात् - from, as a result of प्रणिधान - fixing/placing [attention], profound religious meditation; प्रणिधान is from the Sanskrit root धा - to place. निधान and निधि are also from this root.
वा - or (this 'or' relates 1.23 to the previous four sutras)

*I refer to the original sāṃkhyakārika. Subsequently this philosophy was reinterpreted and re-presented in the epics etc. with theistic content.  Read more about it here: http://en.wikipedia.org/wiki/Samkhya or if you're really keen and money is not an issue, get a copy of the sāṃkhyakārika from mlbd.com --> http://mlbd.com/BookDecription.aspx?id=14602